std::pow implementation

[Gabriel Dos Reis]

Richard Guenther <rguenth@tat.physik.uni-tuebingen.de> writes:

| It has been shown that the current gcc unroller is not able to
| unroll the while loop in std::__cmath_power() and such even inlining
| does not help to create good code. Also inlining the loop if it is
| not unrolled and its contents not sufficiently cprop'ed is harmful.

If you find that after inlining, the status of "constant expression"
is not recognized then I'd encourage you to fill a PR.

-- Gaby